Epidemiological studies have suggested gender differences in the development of hypertrophy and heart disease, but the mechanism and the role of estrogen receptor subtypes is not established. The goal of this study was to determine the role of estrogen receptor subtypes in the development of pressure overload hypertrophy in mice. We performed transverse aortic constriction (TAC) and sham operations in male and female wild type (WT) mice and mice lacking functional alpha estrogen receptor (alpha-ERKO) and mice lacking beta estrogen receptor (beta-ERKO). Body, heart, and lung weights were measured 2 weeks post surgery. WT male mice subjected to TAC showed a 64% increase in heart to body weight (HW/BW) ratio compared to sham, and WT males have increased lung weight at 2 weeks. WT female mice subjected to TAC showed a 31% increase in HW/BW compared to sham, which was significantly less than their male counterparts and no evidence of heart failure. alpha-ERKO females developed a HW/BW ratio nearly identical to that seen in WT littermate females in response to TAC, indicating that the alpha estrogen receptor is not essential for the attenuation of hypertrophy observed in WT females. In contrast, beta-ERKO females responded to TAC with a significantly greater increase in HW/BW ratio than WT littermate females. beta-ERKO females have lower expression of lipoprotein lipase at baseline than WT or alpha-ERKO females. These data suggest an important role for the beta-estrogen receptor in attenuating the hypertrophic response to pressure overload in females. We also examined male-female difference in an ischemia-reperfusion model of injury. Under hypercontractile conditions associated with increased intracellular calcium, male hearts show enhanced ischemia/reperfusion injury compared to female hearts. Our aim in this study was to identify the specific estrogen receptor involved, and the mechanism responsible for this gender difference. Following brief treatment with isoproterenol,a b-adrenergic receptor agonist, isolated mouse hearts were subjected to ischemia and reperfusion. Postischemic contractile function and infarct size were measured in wild-type (WT) male and female hearts, and female hearts lacking functional alpha estrogen receptor (alpha-ERKO), or the beta estrogen receptor (beta-ERKO). WT males exhibited significantly less functional recovery and more necrosis than WT females. Alpha-ERKO females exhibited ischemia-reperfusion injury similar to that observed in WT females, whereas ?ERKO females exhibited significantly lessfunctional recovery and more necrosis than WT females and were more similar to WT males. These data suggest that estrogen, through the beta-estrogen receptor, plays a role in the protection observed in the female heart. Furthermore, we performed DNA microarray analysis to identify genes that were differentially expressed in beta-ERKO female hearts compared to alpha-ERKO and WT female hearts, and found altered expression of a number of metabolism genes in beta-ERKO females compared to alpha-ERKO and WT females, which may be important in ischemic injury.